ILO estimates that there are 21 million victims of forced labour in the world. Photograph: Anupam Nath/AP

What do pharmaceutical firms, defence ministries and big international banks have in common? Each has a massive research budget and places a great deal of value on data.

Data is essential to understanding your operating environment and a fundamental building block towards success. Think of the fight against HIV: no efficient prevention measure or treatment could be designed before the virus was identified and analysed.

The same reasoning applies to the fight to end forced labour and slavery. We must understand more about its operating environment, the victims and the perpetrators to be able to design and implement better prevention and protection measures, increase prosecutions, and to assess whether the millions of pounds invested in programmes to combat forced labour and human trafficking has been money well spent. Only solid and detailed data can provide an answer to this question.

So why don't we have this crucial data? To start with there is a lack of international consensus on the concepts and definitions of what we are dealing with. While overlapping to a great extent, there are variations in the definitions of forced labour, slavery, slavery-like practices, and trafficking. Here at the ILO we use the term forced labour to cover most instances of these coercive practices - with the exception of trafficking for the removal of organs, forced marriage and adoption, unless it leads to a situation of forced labour. Yet it remains that the language we use in the sector is confusing and leaves it unclear what exactly it is we are trying to measure.

There are no shortage of reports, testimonies, government and NGO statistics on victims identified and assisted. While these and other sources often provide useful information and insights, they cannot be extrapolated to provide a reliable picture of the scale and nature of modern forced labour. More often than not, they provide a microscopic focus on a particular aspect of the problem, rather than a bird's eye view. Among the reasons why we have only this narrow view of what we're facing is the lack of funding for serious research and the lack of coordination among institutions. Research would also benefit from an increased engagement of and collaboration with the academic world.

Despite the lack of data there are many different figures on slavery in the public domain. Two new global estimates of slavery/forced labour were published last year: the ILO estimated that there are about 21m victims of forced labour, while Siddarth Kara put the global number of slaves between 22.4m and 30.5m.

Other figures based on probabilistic sampling providing precise data and accurate analysis have also been released but these are limited to one region – the UNIAP's 2009 survey on trafficking among deportees on the Thailand – Cambodia border for example, or one sector – such as an ILO survey on the dry fish industry in Bangladesh in 2010, or one population group – Moldova's 2008 survey on forced labour of migrant workers.

What does all this tell us about modern slavery? That it exists everywhere, and that there is no such thing as a typical victim. Men, women, boys and girls of all ages, social backgrounds and levels of education, are vulnerable. Some are exploited in their place of origin, while many are entrapped thousands of miles from their home. All economic sectors, in both formal and informal economies, can be affected. All victims are subjected to threats and coercion in order to exploit their labour.

But these topline figures do not give us the full picture. There is much that we do not yet fully grasp, and need to understand better if we are to design more effective policies to combat forced labour. We need to know more about trends, underlying causes, people most at risk, the impact of forced labour on victims and on the profile and motivation of their exploiters. An economic analysis could give us more robust information on the profits made and on the losses incurred by individual workers, their communities and countries of origin. Donors and practitioners need to know what works and what does not in the fight against slavery. Rigorous impact evaluations, which require costly data collection before and after interventions, comparing the situation of target and control groups, are almost non-existent in this field.

The ILO's Special action programme to combat forced labour, created in 2002, has confronted many challenges in its efforts to develop and implement rigorous data collection methods. These include ethical problems – for example how to interview current or former victims without jeopardising their safety or wellbeing. There are also technical challenges such as how to design samples and find enough victims to extrapolate the results when the population is so often hidden or isolated and political obstacles around getting governments to engage and not ignore this as a major problem. There are also the inevitable financial constraints and the questions about whether it is justified in investing donor funds in data collection rather than assisting the victims.

So where do we go from here? Much more of everything is needed – especially in terms of cross-sector collaboration – so that national and international institutions are better able to exchange data, and to share and compare their methods and results.

Only then will practitioners be able to make use of the data produced by the global community of researchers ultimately benefiting those who need it – the victims of slavery and those at risk.